The Koiter methodology is a reduced-order model that can predict the initial post-buckling characteristics of structures. Incorporating the Koiter approach within finite element simulations resulted in challenges. These included mesh sensitive initial post-buckling coefficients, computationally expensive high-order derivatives, and phenomena such as locking, which led to unrealistic behaviour of structures. The displacement-based Koiter methodology is introduced to overcome these issues due to the clear connection between the theory and the implementation. The objective of this thesis is to enhance this approach by introducing imperfections, which enables the analysis of initial post-buckling characteristics of imperfect structures. The preliminary results show that the inclusion of imperfections was successful for a plate with linear pre-buckling, but more verification is required. If the inclusion is verified to be successful the current displacement-based approach can be extended to cylinders and used in sensitivity studies to establish guidelines for an imperfection-insensitive cylinder.